Long-duration plasma operation in WEST

The WEST superconducting tokamak, featuring a full tungsten environment and equipped with an actively cooled ITER-grade divertor[1], provides valuable inputs for future ITER operation. Substantial progress has been achieved in developing quasi non-inductive scenarios, resulting in 6 minute-long plasma discharges and 1.15 GJ injected/extracted energy, featuring good L-mode confinement (H_98y,2 ~ 1.0) and a stationary central electron temperature of ~4keV. The plasma duration was limited by outgassing from far-off in-vessel elements when approaching the gigajoule range, with the evidence of a progressive conditioning as long pulses were repeated. A high fluence campaign has been carried out in attached divertor condition (T_e target ~ 20 eV), cumulating 3 hours of plasma and a divertor particle fluence equivalent to an ITER shot. However, the accumulation of redeposited layers on the divertor triggered impurity events, increasingly impacting the plasma operation [2]. In terms of impurity contamination, a resilient radiative fraction is observed due to the strong connection between tungsten sources and radiative losses [3]. Mitigation strategies include wall conditioning using an impurity powder dropper [4] and developing X-point radiator (XPR) regimes. Stable XPR was successfully feedback-controlled for over 10s in L-mode and displayed improved confinement [5]. These approaches show promising prospects for extending this divertor-preserving regime, crucial to reactor operation, towards long durations and improved performance. Furthermore, WEST has demonstrated Ion Cyclotron (IC) assisted breakdown in low electric field conditions (V_loop ~ 0.5V/m), approaching ITER value (V_loop ~ 0.33V/m). IC wall conditioning experiments have also started and will be pursued in the next experimental campaign.